This invention relates to scanning apparatus for monitoring telephone subscriber lines. More particularly, it is concerned with apparatus for scanning telephone subscriber lines to detect supervisory signals and to detect and count dial pulse signals.
In telephone communication systems having common control systems it is the usual practice to separate the detection of supervisory information on the on hook or off hook status of subscriber lines from the signaling information as provided for example, by dial pulse signals. Supervisory detection in these systems is performed by monitoring the conditions of slow releasing holding relays, or their electronic equivalents, and reporting the state of each holding relay to the common control. The scanner in systems of this nature performs only a sensing and reporting function, and the supervisory condition of every line (either on hook or off hook) is reported to the common control on each and every scanning cycle.
Dial pulse signaling information is detected by connecting a subscriber line to one of a set of pooled dial pulse register-senders. A register-sender is connected to a line only during the signaling portion of a call. After the complete address in dial pulses has been received, the dial pulse register-sender is released and additional dial pulse signaling by the subscriber is not possible. Since certain types of switching equipment begin outpulsing immediately after circuit seizure, some exchanges employ a special matrix stage called bylinks to connect incoming lines to dial pulse register-senders in order to minimize the connection delay and insure that all dialed digits are detected properly. In some exchanges, rather than employing the bylink arrangement, the first digit or digit pulse is stored to provide sufficient time for a path through the switching matrix to be established.
In common control systems as described the supervisory condition of each subscriber line is reported on during each scanning cycle regardless of whether or not a change has occurred. The input-output channel between the subscriber network and the common control network thus carries large amounts of redundant data. In addition the common control wastes processing time analyzing the redundant data. Systems of this type also require a pool of dial pulse register-senders to detect and process the dial pulse signaling. In addition, in order to insure the detection of all dial pulse signals from certain types of switching equipment either a special bylink matrix or arrangement for storing the first dial pulse information is required.
Furthermore, in systems of the type described the dial pulse register-sender employed during the address portion of a call is released after the dial pulse information has been received. Thus, no capability is provided for the subscriber to signal the exchange once the address information has been dialed.